You multiply 86 by 42.5 because each board is 42.5 and there are 86 boards.
9 is the base of the triangle 4 is the leg and 5 is the
The area of a trapezoid is basically the average width times the altitude, or as a formula:
Area = h ·
b 1 + b 2
2
where
b1, b2 are the lengths of each base
h is the altitude (height)
Recall that the bases are the two parallel sides of the trapezoid. The altitude (or height) of a trapezoid is the perpendicular distance between the two bases.
In the applet above, click on "freeze dimensions". As you drag any vertex, you will see that the trapezoid redraws itself keeping the height and bases constant. Notice how the area does not change in the displayed formula. The area depends only on the height and base lengths, so as you can see, there are many trapezoids with a given set of dimensions which all have the same area.
